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RED Calc Free Help Pitot Tube Airflow
Pitot Tube Airflow Tool User Guide

Pitot Tube Airflow Tool
Red Calc Tool User Guide

What this tool can do for you

This tool, along with a pitot tube and a digital manometer, allows you to accurately measure the airflow within a duct of 4 inches (10.2 cm) or greater, such as that of a kitchen range hood or dryer vent.

It may be used with a standard steel pitot tube, such as a Dwyer Series 160 (pictured below) or with an averaging flow sensor pitot tube, such as the Dwyer Series PAFS-1000.

Because ASHRAE 62.2 requires the measurement of existing and installed ventilation fans, the pitot tube is a useful device to have at hand. Additionally, for the sake of determining possible back drafting of vented combustion appliances, it is important to be able to measure the flow of clothes dryers, rather than estimating their airflow.

In order to find airflow from velocity pressure, calculations are required; this is what this Pitot Tube Airflow tool will do for you. First, it helps you calculate the air velocity based on pitot tube measurements and air density. Then it calculates the corresponding airflow in the duct by helping you determine the duct cross-sectional area.

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Standard Pitot Tube

Calculated values

     Primary Results
  • Airflow rate.
  • 90% Airflow rate - many pitot tube manufacturers recommend using only 90 percent of the measured airflow rate if only one pitot tube measurement is taken at the center of the duct.

     Secondary Results
  • Atmospheric pressure.
  • Air speed.
  • Calculated cross-sectional area.

Tips

  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs). Read more.
  • When you purchase a pitot tube, it is important to read and retain the instructions. When setting up your pitot tube, always follow the manufacturer's recommendations.
  • You will find that some pitot tube manufacturers list a correction (K) factor in their instructions. We have included a place to enter this coefficient in the Pitot Tube Airflow Tool as the second input; "Pitot tube correction (K) factor".
  • As a separate issue from the correction (K) factor, some pitot tube manufacturers recommend multiplying the airflow result by 90 percent (0.9) if you take only one measurement at the center of the duct. The Pitot Tube Airflow Tool gives this adjusted airflow as its final result.
  • When you connect your pitot tube to your manometer with the appropriate hoses, make sure you attach the hoses to your manometer so that you are subtracting the static pressure (the smaller pressure) from the total pressure (the larger pressure). If you are getting a negative number on you differential manometer, reverse the hose setup.
  • A pitot tube of 8 to 12 inches (20 to 30 cm) in length is ideal for residential flow measurements.

Inputs and field measurements

  • Velocity pressure - the reading from the manometer connected to the pitot tube.
  • Pitot tube correction (K) factor - a value from the manufacturer for a particular pitot tube model. If no correction (K) factor is stated by the manufacturer, use the default value of 1.
  • Duct air temperature - the temperature of the air being measured with the pitot tube. This affects the density of the air.
  • Altitude - the distance below or above sea level. This affects the density of the air.
  • Duct cross-sectional type - select the appropriate option of Round, Rectangular, Oval, or Other.
    • Diameter, Width, Height, Long diameter, or Short diameter - the inputs needed to calculate the Cross-sectional area of the duct being measured.

Background

The pitot tube was invented by Henri Pitot, a French engineer, in the 18th century. This ingenious device solved the problem of not being able to directly measure the velocity pressure of airflow. This is accomplished by subtracting the measured static pressure from the measured total pressure to find velocity (dynamic) pressure [total pressure - static pressure = velocity pressure]. The device is very useful for measuring airflow in a dryer duct or the duct of a ventilation fan.

The static or bursting pressure in a duct "pushes" against the walls of the duct, similar to what happens to the wall of a balloon when you blow it up. The velocity pressure is created by the speed of the airflow and its density. This is similar to what happens when you let the valve of a blown-up balloon go; the static pressure is converted to velocity pressure as the balloon flies through the air.

It is interesting to note that pitot tubes are a common protrusion from the fuselage of airplanes as an air-speed measurement device.

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Positioning pitot tube in duct

Best practices

To ensure accurate velocity pressure readings, the tip of the pitot tube must be pointed directly toward the oncoming airstream.

When the pitot tube is correctly aligned, the velocity pressure reading will be at or near its maximum. Because accurate readings cannot be taken in a turbulent airstream, the pitot tube should be inserted at least 8-1/2 duct diameters downstream (after) and 1-1/2 duct diameters upstream (before) any elbows or other fittings/obstructions that cause turbulence. If this is not possible, do your best to insert the pitot tube in a smooth and straight duct section.

If circumstances do not permit you to average various readings from various locations in the duct (called a traverse), take one reading in the center of the duct and multiply the result by 0.9. This multiplication is done for you in the last result of the RED tool.

In order to make accurate readings, the pitot tube must be kept free of dust and debris.

Related tools

  • ASHRAE 62.2-2013 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
  • ASHRAE 62.2 California: Determine whole-building ventilation requirements for new and existing dwellings with the special California version.
  • ASHRAE 62.2-2010 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
  • Depressurization Analysis: With this "solve-all" tool you can calculate house pressure, CFM50 tightness limit (Depressurization Tightness Limit), or total exhaust fan flow.
  • Box Airflow: Determine the flow of an exhaust fan with this tool, a cardboard box, and your digital manometer.

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